1. Field of the Invention
The present invention relates to a current mirror with bipolar transistors, working with high precision even if the transistors are very low gain transistors.
It is known that bipolar transistors have characteristics that change with the conditions of use, or even during manufacture. In particular the gain, in current, decreases when the temperature diminishes, or under the effect of a radiation of light or particles. The loss of gain leads to an intrinsic copying error in the current mirrors.
2. Description of the Prior Art
A current mirror is an assembly, such as the one shown in FIG. 1, which enables the enforcement, through a second arm, of a current I.sub.0 which, except for errors, is identical to the current I.sub.1 that flows through a first arm. The first arm comprises a current source 1, a transistor 2, the collector of which is connected to the base, and a negative feedback resistor 3. The second arm comprises a transistor 5 and a negative feedback resistor 6. The bases of the two transistors 2 and 5 are joined in such a way that the current I.sub.1 which flows in the first arm controls the current I.sub.0 enforced through a load 7 in the second arm.
This type of simple current mirror suffers from an intrinsic copying error which depends on the gain of the transistors. Indeed, for a simple mirror with a gain equal to unity, the transistors 2 and 5 of which are matched in terms of V.sub.BE (base-emitter voltage) and the negative feedback resistors 3 and 6 of which are matched, the error in the gain of the mirror is expressed through the equation: EQU I.sub.0 =I.sub.1 [1-2/(.beta.+2)]
.beta. being the gain of the transistors, the same for the two transistors since they are assumed to be identical and under the same conditions of bias. The relative copying error is equal to -2/(.beta.'2) and, in most applications, with transistors having a gain that is far greater than 1, this error is not the main cause of any observed lack of imprecision, and it remains masked by the offset voltage of the pair of transistors or the non-matching of the negative feedback resistors 3 and 6. However, as soon as the gain of the transistor decreases, for any reason, the error due to the low gain (.beta.&lt;1) becomes predominant. Indeed, it is seen that the gain .beta. comes into play linearly and at the denominator of the equation, in such a way that, when the gain tends towards zero, the error tends towards -100%.
The current applications of electronics make it necessary, however, to have mirror copying precision of over 10% which can be expected with transistors that have undergone constraints, having a low gain, for example of 1 to 10.
A first known approach is presented by the Wilson mirror shown in FIG. 2. This is the equivalent of a standard mirror in which the amplifier transistor 8 is subjected to negative feedback by the mirror constituted by the transistors 2 and 5. In this figure as in the following figures, the load 7 is no longer shown since it is not a factor in the understanding of the invention.
Assuming that the three transistors have the same gain .beta., the gain error of the Wilson mirror is expressed by a quadratic relationship: EQU I.sub.0 =I.sub.1 [1-2/(.beta..sup.2 +2.beta.+2)]
A second known approach lies in the buffered mirror shown in FIG. 3. In this assembly, the transistors of the master and copying arms, respectively 2 and 5, have their base currents not tapped directly from the source I.sub.1 as in the case of FIG. 1, but through an amplifier transistor 9 the base of which is connected to the source I.sub.1 and the emitter to the two bases of the transistors 2 and 5, the collector of this transistor 9 being supplied with a draw-back volta V.sub.R. The error is given by: EQU I.sub.0 .perspectiveto.I.sub.1 [1-2/(.beta..sup.2 +.beta.+2)]
For transistor gains far greater than 1, the error that is introduced by this Wilson mirror and this buffered mirror has the shape -2/.beta..sup.2 and gives a very substantial improvement of the simple mirror: for .beta.=100, the error goes from -2% to -0.02%, which becomes negligible. However the effect of the quadratic law diminishes when the gain of the transistor becomes close to or below 1. For example, the Wilson mirror has an error of the order of -8% for a gain of the transistors equal to .beta.=4.